1. Field of the Invention
The present invention relates to a mounting structure for mounting a member required to be positioned with high precision, for example, a rail for a linear guide bearing used in industrial equipment, on an upper surface of a base member or the like. More particularly, the invention concerns improvements in screw holes bored in a translation apparatus or a mounting member for the translation apparatus or the like, and in screw holes for fixing thereof bored in a housing of a bearing or the like.
2. Related Background Art
For fastening two members to each other with a bolt or a screw, it is common practice to carry out a work of removing burrs and the like appearing around the screw hole for the bolt or the like to be screwed in. For example, when a prepared hole for formation of the screw hole is bored in a stationary member or the like for a rail in the linear guide bearing to be mounted thereon, small protrusions appear in the periphery of the prepared hole. These protrusions are removed using oil stone or the like.
A specific example will be described based on a case wherein a member to be fixed is the rail for the linear guide bearing and a stationary member is the base member for fixing of the rail and wherein the rail is fastened to the base member with high position precision. FIG. 9 shows the appearance of the linear guide apparatus as a linear guide bearing and FIG. 10 is a cross-sectional view thereof in the direction of arrows along line 10—10 in FIG. 9 to show the internal structure thereof. As illustrated, the linear guide apparatus is comprised of a rail 2 extending in the axial direction and having roller grooves 57 in outside surfaces thereof so as to receive rolling elements or balls 51 in a rotatable state, and a slider 60 having circulation paths 44 for the balls 51 inside and being held so as to be slidable on the rail 2 through the balls 51 in roller grooves 47 opposite to the roller grooves 57. A plurality of balls 51 roll in the circulation paths 44 and between the two roller grooves 57, 47. The slider 60 is composed of a slider body 41 having straight circulation portions 45 of the ball circulation paths 44, and end caps 42 having their respective curved circulation portions 46 to form the ball circulation paths 44 as endless tracks when the end caps 42 are in communication with the straight portions 45 and the spaces between the two roller grooves 57, 47. In the case of the base member to which the rail 2 is fastened, the removing work with the oil stone or the like is normally carried out to remove the protruding portions appearing in the periphery of the holes during boring of the prepared holes before formation of the screw holes for fastening in the base member. After that, the screw holes are formed in the base member, and it is also common practice to carry out the removing work of removing burrs and the like appearing in the periphery of openings of the screw holes during tapping thereof or the like.
The rail is provided with concentric counterbores and pass holes (through holes) for the bolts, which fix the rail to the base, and the inside diameter of the pass holes is greater than the nominal diameter of the screw holes bored in the base. In this structure, the rail is positioned against the base by making use of the difference between the outside diameter of the bolts and the inside diameter of the pass holes, and thereafter the bolts are tightened to fix the rail onto the base.
In cases wherein two or more members are fastened to each other with a bolt or a screw, a specific member is provided with a screw hole, while the other member(s) is provided with a through hole through which the bolt or the like penetrates. The diameter of the through hole is normally set greater than the nominal diameter of the screw hole in order to absorb a positional error of the screw hole or the through hole. When the members are fastened to each other with the bolt or the like through these screw hole and through hole, the screw causes a lifting force toward the through hole to act on the peripheral part of the screw hole. Further, this force is enhanced by the role as a fulcrum of a lever at the peripheral part of the through hole, so as to make a protrusion in the peripheral part of the screw hole (see FIG. 8).
If the fastening work is carried out only once and if the members will never be unfastened, the existence of this protrusion will pose no problem at all. However, if the fastening work is carried out repeatedly using this screw hole, there will arise problems because of this protrusion, such as the problem that the member cannot be fixed at the same position, the problem that joint surfaces of the members cannot fit closely, or the like. Therefore, when high precision is required in the fastening work, it is essential to remove such protrusions. Thus the removing work was always carried out even if it took a considerable time. This will be explained with a specific example.
FIG. 7 is a cross-sectional view of a counterbore, a pass hole, and a screw hole in a stationary member (a base member) and a member to be fixed (a rail) placed in the positional relation in an actually fixed state. In FIG. 7, the rail 2 is provided with a pass hole 5, and a counterbore 7 concentric with the pass hole 5 and continuous therefrom. The pass hole 5 permits a bolt B (shown diagrammatically by a dashed line) to reach the base member, and the counterbore 7 houses the head of the bolt to fix the rail 2 to the base member 1 by an urging force from the bottom surface of the head of the bolt against a surface 18. The base member 1 is provided with a screw hole 10 to be coupled with the bolt, at a predetermined fixing position in an upper surface 19 as a fixing surface for the rail 2.
In the illustrated state, the unrepresented bolt is put through the counterbore 7 and pass hole 5 to be screwed into the screw hole 10 whereby the rail and base member are fixed to each other. In this case, the screw thread in the screw part of the bolt exerts the lifting force on the screw thread of the screw hole and the like upward in the screw hole or toward the upper surface 19 of the base member. This causes protruding deformation of the peripheral part 19a of the screw hole in the upper surface 19 of the base member 1. Since this deformation occurs under the leverage by an opening edge 21 of the pass hole 5 of the rail 2, the deformation is further promoted to larger dimensions.
An example of the above deformation is illustrated in FIG. 8. The illustrated example is the result of measurement of a surface profile of the base member under such conditions that the rail for the linear guide bearing was fixed once to the base member with the bolt of M8 and thereafter the bolt and rail were detached. As illustrated, it is apparent that the protrusion of about 10 μm appeared in the region about 0.5 mm wide around the screw hole having the nominal diameter of 8 mm.
When the rail 2 is fixed to the base member 1 in practice, there can be cases wherein the tightening work of bolt is carried out several times and fine adjustment of the fixing position of the rail is repeated before final fixing. However, when the high precision of ten micrometers or less is required, it may be practically impossible to fix the rail at the predetermined position with precision because of a protrusion as described above.
There can also be cases wherein the high position precision of the rail is achieved by only one tightening work of bolt. Even in such cases, however, there were possibilities that the rail had to be dismounted after fixing of the rail and there could arise the problem that it became difficult to fix the rail again at the correct position on that occasion, and the like. Further, there was the problem that this protrusion became greater with increase in the number of tightening works of bolt and the mounting precision degraded with increase of the dismounting and mounting works of the rail.
It was common practice heretofore to carry out the removing work of the protrusion as occasion arose, i.e., when the size of the protrusion posed a practical problem. Such practice involved such problems as the problem that the removing work required a considerable time, the problem that there were cases wherein it was difficult to carry out the removing work, and so on.